Wireless communication eliminates the burden of wiring work for wired communication of the past, and is additionally catered for usage as a technology that realizes mobile communication. For example, IEEE (The Institute of Electrical and Electronics Engineers) 802.11 may be cited as an established standard regarding wireless LANs (Local Area Networks). IEEE 802.11a/g is already widely prevalent.
With many wireless LAN systems such as IEEE 802.11, an access control protocol based on carrier sense such as CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) is implemented, with each station being configured to avoid carrier collisions during random channel access. In other words, a station that has produced a transmission request first monitors the medium state for a given frame interval DIFS (Distributed Inter Frame Space), and if no transmitted signal exists during this space, the station conducts a random backoff. In the case where no transmission signal exists in this space as well, the station obtains an exclusive channel usage transmission opportunity (TXOP), and is able to transmit a frame. Also, “virtual carrier sensing” may be cited as a methodology for resolving the hidden terminal problem in wireless communication. More specifically, in the case where duration information for reserving the medium is stated in a received frame not addressed to the receiving station, that station predicts that the medium will be in use for a period corresponding to the duration information, or in other words virtually senses the carrier, and sets a transmission pause period (NAV: Network Allocation Vector). In so doing, channel exclusivity during a TXOP is assured.
Meanwhile, with the IEEE 802.11a/g standard, orthogonal frequency-division multiplexing (OFDM) is used in the 2.4 GHZ band or the 5 GHz band to support a modulation method that achieves a maximum communication rate (physical layer data rate) of 54 Mbps. Also, with the standard's amendment IEEE 802.11n, MIMO (Multi-Input Multi-Output) communication methods are adopted to realize even higher bit rates. Herein, MIMO is a communication method that realizes spatially multiplexed streams by providing a plurality of antenna elements at both the transmitter end and the receiver end (as is commonly known). Although high throughput (HT) exceeding 100 Mbps can be achieved with IEEE 802.11n, even greater speeds are being demanded as the information size of transmitted content increases.
For example, by increasing the number of antennas on a MIMO communication device to increase the number of spatially multiplexed streams, throughput for 1-to-1 communication can be improved while maintaining backwards compatibility. However, improvements in per-user throughput for communication as well as in throughput for multiple users overall are being demanded for the future.
The IEEE 802.11ac Working Group is attempting to formulate a wireless LAN standard whose data transfer rate exceeds 1 Gbps by using the frequency band below 6 GHz. For its realization, space-division multiple access methods whereby wireless resources on a spatial axis are shared by a plurality of users, such as multi-user MIMO (MU-MIMO) or SDMA (Space-Division Multiple Access), are effective.
For example, there has been proposed a communication system that combines the two technologies of carrier sensing in the legacy IEEE 802.11 standard and space-division multiple access with an adaptive array antenna by using RTS, CTS, and ACK frames in a frame format that maintains backwards compatibility with the legacy 802.11 standard (see PTL 1, for example).
Also, with IEEE 802.11n, an RD (Reverse Direction) protocol is adopted in order to make data transmission in an exclusive channel usage period (TXOP) more efficient. With an ordinary TXOP, only one-way data transfer is conducted wherein the station that has obtained an exclusive channel usage right transmits a data frame. In contrast, with the RD protocol, two roles called the RD initiator and the RD responder are defined. As a result of the RD initiator indicating an RDG (RD Grant), or in other words permitting or granting reverse data transfer, in a specific field in a MAC (Media Access Control) frame sent by the RD initiator (downlink), the RD responder is subsequently able to transmit a data frame in the reverse direction (uplink) addressed to the RD initiator in the same TXOP (see PTL 2, for example).
At this point, a communication system conducting space-division multiple access can improve throughput for multiple users overall (discussed above), but it is thought that spatially multiplexed frames in a TXOP can be more even more efficient by applying the RD protocol defined in IEEE 802.11n.
However, consider a practical configuration wherein an access point takes the role of an RD initiator, and a plurality of terminals take the role of RD responders, for example. In this case, when data frames are sent from the plurality of terminals plurality of terminals to the access point by uplink, the access point will be unable to separate users unless the respective stations multiplex their frames at the same time.
Also, in the case of applying space-division multiple access to a wireless LAN, the case of multiplexing variable-length frames on the same time axis is conceivable. However, if the lengths of the frames sent from respective stations differ, then the received signal power at the access point will vary drastically as the amount of frame multiplexing increases or decreases. This induces unstable operation with respect to automatic gain control (AGC), and there is also a possibility of problems occurring from various perspectives, such as the power distribution in a frame becoming no longer constant with respect to the RCPI (Received Channel Power Indicator) standardized in IEEE 802.11.
In short, a plurality of RD responders requires frames to be sent to an access point at the same time, and additionally requires it to be configured such that even if a plurality of frames with different lengths are sent from an upper layer, the lengths of the frames ultimately sent from the PHY layer are made to be uniform.